In conventional cellular communications, if a terminal wants to access a cellular network, the terminal needs to complete a random access process. The random access process mainly includes interaction between a terminal and a base station, to implement synchronization with the base station. In addition, the base station allocates resources to users. In Long Term Evolution (LTE for short), a terminal is notified of a random access resource by using a system message or mobile control information. Different systems may use different random access resource configurations. Specifically, a system may configure a frequency domain location of a random access resource and a frame or a subframe in which a random access resource is configured.
Contention-based random access and non-contention based random access are supported in LTE. The contention-based random access means that multiple terminals use a same preamble and perform random access on a same time-frequency resource. The non-contention based random access means that the system specifies dedicated preambles for terminals, and when the terminals access the system, collision does not occur. In an LTE system, the contention-based random access is generally completed by using four steps. FIG. 1 is a contention-based random access process, as shown in FIG. 1.
Step 1: A user terminal randomly selects a preamble, and sends the preamble to a base station on a random access channel (RACH for short) by using a message 1.
Step 2: The base station detects the preamble, and sends a random access response (a message 2) to the user terminal, where the random access response includes the following information: location indication information of an uplink resource allocated to the user terminal, and a temporarily-allocated cell radio network temporary identifier (C-RNTI for short).
Step 3: After receiving the random access response, the user terminal sends an uplink message (a message 3) on the allocated uplink resource according to indication of the preamble.
Steps 4: The base station receives the uplink message of the user terminal and returns a contention resolution message (a message 4) to a user terminal that successfully performs accessing.
In the LTE, a group of 64 preambles in total are defined, and a preamble is a group of zero correlation code. Preambles for contention are categorized into two groups: a group A and a group B, and a quantity of preambles in the group A is determined by a parameter preamblesGroupA. If the quantity of preambles in the group A is equal to a total quantity of preambles for contention, it means that the group B does not exist, and a preamble is selected from the group A. A preamble requires relatively low synchronization precision. The base station estimates a timing advance (TA for short) of the user terminal according to the received preamble, so as to adjust uplink transmission timing of the user terminal.
If the base station can correctly demodulate the preamble, the base station sends the random access response to the user terminal. The random access response message includes the TA and an uplink grant (UL Grant) resource, and the UL grant resource is an uplink transmission resource of a corresponding size determined according to the sent preamble. The user terminal transmits the message 3 by using the UL grant resource.
US20140071954A1 discloses a method for adaptive transmission time intervals (TTI for short), and is mainly to adapt to various different service requirements in the future. For example, some services have a relatively low transmission delay, and some services have a relatively high transmission delay. Therefore, US20140071954A1 defines a frame structure that can adapt to different services, and multiple different TTIs can be supported at the same time to adapt to different services in the future.
In a frame structure disclosed in US20140071954A1, one band is divided into multiple sub-bands or carriers, and a frame structure of a TTI of a length is transmitted on each sub-band. The sub-bands are multiple small bands that are obtained by dividing a large band, and there is no band leakage guard band between the bands, or only an OFDM symbol is used to eliminate interference between the bands. In a broad sense, a sub-band may be an independent small band or carrier. Generally, information about a TTI supported in a system is notified to a terminal by using a system broadcast message. Generally, a notification message is sent only on a sub-band. Therefore, there is a sub-band used for sending a downlink broadcast message. The sub-band in US20140071954A1 is referred to as a common sub-band, and is mainly used to send the downlink broadcast message, for example, an MIB or SIB. Similarly, to save random access resources, random access resources are not defined on each sub-band. For example, random access resources may be defined on the common sub-band.
An existing LTE random access process is completed on one carrier (a random access process is also completed on one carrier in a carrier aggregation scenario), and a process is shown in FIG. 1. After the message 3 is transmitted, the base station sends a contention resolution message. After contention is completed, the random access process is also completed, and immediately, a data transmission process is entered.
In LTE, only a single frame structure is supported, a same TTI is used, and a TTI of 10 ms is currently defined. A length of a TTI may be 1 ms in the future, and the prior art cannot satisfy a scheduling requirement of a 1 ms delay in the future. In addition, if multiple TTIs are supported in the future, the prior art cannot satisfy a scheduling requirement of multiple TTIs in the future.